Generally zinc coating is used in various fields as an anti-corrosive treatment for steel products. Recently, steel plates coated with high corrosion-resistant zinc-alloys, prepared with addition of Al, Mg and other elements to Zn, are also used, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-240947 and No. 10-306357. These coated steel plates are first coated and then converted into final products by applying various working processes. Therefore, they have uncoated parts in, for example, cut ends and welded parts.
In environments with moderate corrosiveness, cut ends and welded parts are not corroded heavily. Meanwhile, steel surfaces coated with zinc-alloys providing greater corrosion-resistance than conventional coatings remain uncorroded for long periods of time, with the result that red rust formed by corrosion of cut ends and welded parts become conspicuous in the course of long use.
Therefore corrosion protection must be provided to such parts. Corrosion protection is provided by spraying or other methods that provide appearance similar to that of metal coating. However, limited application of spraying only to cut ends involves many problems in terms of equipment and cost. Common anti-corrosive paintings are sometimes used. However, ordinary white or red paints spoil the appearance because repaired parts covered with such colors are widely different from the surface covered with zinc-alloy coatings.
Therefore, zinc-rich paints containing zinc powder are often used. Zinc-rich paint coating is a commonly used method specified in the JIS Standards (JIS-K-5553). As with zinc coating, however, use of Mg, Al and other alloy elements, together with zinc powder, will probably provide greater corrosion-resistance.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2001-164 discloses a zinc-rich paint prepared with addition of an alloy powder comprising Mg of 1 to 10%, Al of 2 to 19% and Zn as the remainder. Japanese Unexamined Patent Publication (Kokai) No. 11-343422 discloses an organic rust-preventing paint containing flaky zinc-alloy powder comprising Al of 5 to 10 mass %, Mg of 0.01 to 5 mass % and Zn as the balance.
These technologies featuring the addition of powder of alloy elements are intended for providing corrosion-resistance to the surface of steel products.